1. Field of the Invention
The present invention relates, in general, to a variable-regeneration directional control valve used for controlling the flow of operating fluid relative to an actuator, such as a boom, arm or bucket cylinder, in a hydraulic circuit of a construction vehicle and, more particularly, to a structural improvement in such a valve for effectively selecting the return fluid regenerating function by an outside control signal.
2. Description of the Prior Art
In a hydraulic circuit for construction vehicles such as power excavators, "regeneration of fluid" means that return fluid in a return line extending from an actuator is fed back to a fluid supply line extending to the actuator, thereby preventing the generation of cavitation in the fluid supply line due to a shortage of pressurized fluid in the fluid supply line and smoothly regulating the moving speed of the actuator.
In order to regenerate the return fluid from an actuator in a construction vehicle's hydraulic circuit, it must be possible to selectively operate the actuator by the weight of an associated working member in place of pressurized fluid output from a hydraulic pump. For example, in the operation of a power excavator, return fluid under high pressure is obtained by the weight of a boom during a boom-down motion and in turn is used in a boom-up motion.
FIG. 1 is a view showing the construction of a directional control valve or arm control valve, which is used for controlling the flow direction of fluid for an arm cylinder and has a typical regeneration circuit. FIG. 2 is a circuit diagram showing the construction of a hydraulic circuit provided with the regeneration arm control valve of FIG. 1.
As shown in FIGS. 1 and 2, the arm cylinder ARM is connected to a hydraulic pump P through two fluid lines 101a and 101b. An arm control valve CV is mounted to the lines 101a and 101b. In the arm control valve CV, a spool linearly moves in a valve body in response to an operator's control signal 11, 31, thereby switching the internal lines of the valve CV and controlling the flow direction of fluid, which is supplied from the pump P to the arm cylinder ARM. The arm control valve CV thus starts or stops the arm cylinder ARM and controls the moving direction of the cylinder ARM. When the valve CV is switched into the neutral position, the internal lines except for the center bypass line 103 are closed, so that the pressurized fluid output from the pump P does not flow to the arm cylinder ARM but returns to the tank 53 through the bypass line 103.
In the regeneration arm control valve CV for the arm cylinder ARM repeatedly operated by the weight of the arm, a regeneration passage R is connected between a return passage CH and a tank passage T inside the valve block of the valve CV. Two lines 45 and 51 connect the regeneration passage R to the tank passage T. In a weight operation of the arm cylinder ARM, an orifice effect of an appropriate orifice cross-sectional area is formed between the above lines 45 and 51 thereby forming a pressure in the second fluid line 101b. In the above state, return fluid in the return passage CH is partially fed back to the large chamber of the cylinder ARM through a regeneration line 47, check valve 29 and fluid line 48.
The remaining return fluid in the return passage CH returns to the tank T through the lines 45 and 51.
While the arm cylinder ARM is operated by the weight of the arm, the moving speed of the cylinder ARM is influenced by the amount of return fluid. In the prior art, cavitation, which may be formed in the arm cylinder ARM, is thus prevented by forming a back pressure in the return line and by partially feeding the return fluid to the fluid supply line in order to partially regenerate the return fluid.
Particularly in an excavating work of a power excavator, a regeneration circuit and regeneration cancel circuit are selectively used during the operation of the excavator. That is, the pressure in the fluid supply line during an excavating work is increased during an arm-in motion, so that the back pressure caused by the orifice formed between the regeneration passage R and the tank passage T causes a pressure loss in the hydraulic circuit. Therefore, when the pressure or self pressure in the supply line is applied to the piston 24 through the line 49 and is higher than an opposite pressure caused by elasticity of the valve spring 25, the self pressure pushes the regeneration switching spool 22 of the valve CV to the right in FIG. 1. The spool 22 in the above position enlarges the cross-sectional area of the opening between the regeneration passage R and the tank passage T and thereby removes the orifice effect from the valve CV. The back pressure in the return line is thus reduced, thereby reducing the pressure loss in the hydraulic circuit during the excavating work.
However, in the regeneration cancel circuit using self pressure (pressure of operating fluid supplied from the pump P to the supply line of an actuator), the regeneration canceling operation is controlled by the difference of relative pressure between the self pressure applied to the pressure receiving area of the piston and the constant opposite pressure caused by elasticity of the valve spring.
Therefore, it is impossible to control either the regeneration circuit or the regeneration cancel circuit by an outside control signal, so that the regeneration canceling pressure cannot be appropriately controlled even if the pressure loss caused by the back pressure is reduced to an acceptable point.
However, during an excavating work of an excavator, it is preferable to appropriately change the regeneration canceling pressure in order to increase the regeneration canceling pressure. In the above case, the return fluid is forcibly regenerated, thus increasing the amount of the fluid in the supply line and thereby increasing the moving speed of the actuator during the excavating work. In addition, it is necessary to change the regeneration canceling pressure in order to reduce the regeneration canceling pressure. In the above state, the loss caused by the back pressure in the return passage is reduced and increases the excavating power of the excavator.